Infrasound sensors are used in a diverse set of applications ranging from discovering oil pools, finding underground structures, locating seismic events, and even predicting earthquakes or volcanic eruptions. The wide range of sensors employed in these activities includes accelerometers, strain gauges, inertial oscillators, and laser interferometers. Generally, the focus of these systems has been to cover a broad band of infrasound frequencies and to avoid sensitivity to transversely polarized infrasound waves. However, when the characteristics of a signal are known, such as the speed of sound in certain geologic strata, broadband sensors do not perform as well as sensors tuned to the signal.
The shortcomings of the broadband sensors are most apparent in the signal-to-noise ratio of the sensor output signal. All of the energy in the entire sensitivity range of the sensor is measured and the desired signals must be filtered out of those data. Additionally, infrasound signals have such low frequencies that they are hard to transmit after conversion to electronic signals. Accurate transmission of low frequency signals typically involves a modulation scheme by which a higher frequency, which is less susceptible to noise, is used to carry the low frequency information. The present invention is ideal for measuring infrasound signals because it can be tuned for reception of a specific range of signals and because it allows for frequency modulation as the infrasound signal interacts with the magnetic infrasound sensor itself.
Infrasound waves are extremely low frequency sound waves generally below the 20 Hz human hearing threshold and travel at the speed of sound through a medium. In the earth, their speed is typically close to 1460 meters per second. Based on their speed and frequency, infrasound waves typically have a wavelength in excess of 243 meters. Infrasound waves are capable of traveling around the earth, enabling the measurement of distant events if a sufficiently sensitive sensor is used.